There is a trend nowadays for portable radio telephone units to be increasingly lightweight and compact. This in conjunction with the fact that such units are cordless offers not only enhanced mobility but also true portability, so that the user can readily carry a portable radiotelephone unit, for example in a pocket or briefcase, wherever he or she goes. Particularly in the case of cellular radio, this gives the user a very flexible and powerful means of communication, which can always be to hand.
Portable radio telephones necessarily comprise a user-interface including a keypad having a plurality of buttons or keys which can be depressed by the user in order to operate the telephone, i.e. to dial a telephone number or perform other functions such as for example to display a telephone number or other data stored in memory. However the very portability of the telephone apparatus means that it may be placed in locations where objects may interfere with the user-interface mechanism, or indeed the user himself may inadvertently depress one or more of the keys while handling the telephone, thereby activating the number or control keys. Inadvertent actuation of the keys may, for example, turn the apparatus on or off, cause transmission of unwanted signals and prevent use of a radio channel, or cause undesired functions to occur. Hence functions or operations may be accidentally engaged which can reduce the operating life of the battery which powers the portable telephone.
Our co-pending British patent application No. 8919215.7 (Our reference: PAT 89011) concerns a particularly compact telephone which includes a main body and an extending sleeve-like portion mounted for slidable movement between a retracted position when not in use and an extended position for use. The extending portion is adapted to conceal at least some of the buttons or keys when it is in the closed position to prevent accidental actuation of these buttons or keys.
European patent application EP-A-0,346,639 discloses a portable radio telephone including a pivotally mounted `flip` element which contains a microphone and which can be flipped open for use and flipped to a closed position when not in use. In the closed position the flip element covers the keypad to prevent the keys from being mechanically actuated externally. However the keys are also disabled in case any foreign object should become lodged between the keypad and the flip element which might otherwise cause undesired actuation of one or more keys when the flip element is closed.
U.S. Pat. No. 4,670,747 discloses a portable two-way radio comprising an alphanumeric keypad having function keys and a display. The display consists of a series of system related menu icons surrounding an alphanumeric display area. A MENU key is used to select the desired function. The mode selection takes place by repeatedly pressing the special MENU key which causes a cursor to move in a circular fashion from icon to icon. A separate STEP key is then used to select the specific characteristic within the selected function. One of the menu icons corresponds to a keypad lock function. The lock function is selected by repeatedly depressing the MENU key until the cursor is located adjacent the icon corresponding to the keyboard lock. Pressing the STEP key once at this stage activates the keyboard lock. None of the keys (including the MENU key) except for the STEP key itself, will function when the lock is activated. Pressing the STEP key again deactivates the keyboard lock whereafter the MENU key may be pressed repeatedly to exit the lock function and select the desired function.
A drawback with the keyboard lock disclosed in U.S. Pat. No. 4,670,747 is that there is no unique predetermined sequence of keystrokes which activates the lock. On the contrary, the actual sequence of keystrokes required to activate the lock varies depending on which function of the radio is currently selected. Hence the user may have to depress the MENU key any number of times to select the lock function, confirmed by visually checking the cursor position on the display, and then depressing the STEP key to activate the lock. The same sequence of keystrokes does not always activate the lock. Moreover, once the lock has been activated only a single keystroke, i.e. depression of the STEP key, is required to deactivate the lock. This is disadvantageous for a portable radio telephone because it is very easy to accidently activate a single key, viz the STEP key which would release the keyboard lock and thereafter any further accidental actuation of the keys may enable power consuming features which would cause an unnecessary drain on the battery.